1. Field of the Invention
The invention relates to an electrochemical half-cell, in particular for the electrolysis of an aqueous alkali metal chloride solution.
2. Description of Related Art
DE-A-44 44 114 discloses an electrochemical half-cell for the electrolysis of an aqueous alkali metal chloride solution, with a plurality of gas pockets lying above one another, there being a gas diffusion electrode (“GDE”) between each gas pocket and the electrolyte space. The gas diffusion electrodes are fastened and sealed to structural elements of the half-cell with the aid of support elements, which are designed for example as terminal strips. A particular disadvantage associated with a clamping connection is that sufficient sealing of the gas space from the electrolyte space generally cannot be ensured in the long term. Working lives longer than three years are generally necessary for industrial implementation, since economic viability is difficult to achieve otherwise. Furthermore, small pressure surges that occur in the electrolyser can loosen the clamping connection of the GDE. This compromises the integrity of the connection, so that gas from the gas pockets escapes into the electrolyte space or the electrolyte floods the gas pockets.
EP-A-1 029 946 describes a gas diffusion electrode, having of a reactive layer and a gas diffusion layer and a collector plate, for example, a silver mesh. The coating does not completely cover the collector plate, but leaves a coating-free edge protruding. A thin metal plate in the form of a frame, preferably made of silver, is applied to the gas diffusion electrode so that the metal frame covers as small as possible an area of the electrochemically active coating. The frame protruding from the gas diffusion electrode is used for connecting the gas diffusion electrode to the housing of the half-cell, for example, by welding. This method of making contact is complicated and covers up some of the GDE surface, so that the local current density of the free GDE surface is increased and the performance of the electrolyser is reduced owing to a higher electrolysis voltage. The complicated installation furthermore entails high manufacturing costs of the electrolyser.
EP-A 1 041 176 also describes a gas diffusion electrode with a coating-free edge. The gas diffusion electrode in this case is shown connected to the current collector frame of the cathode half-cell by welding in the coating-free edge region. The cavities between two neighboring gas diffusion electrodes are sealed with an alkali-resistant material. A disadvantage of this installation method relates to problems with the sealing material required to obtain sufficient sealing. The sealing effect decreases over the course of operation of the electrolyser, so that the useful life is insufficient terms of economics.
Since the gas diffusion electrode needs to be connected to the electrolyser, a low-impedance connection should typically be ensured, especially for industrial application. Even very minor junction resistances can lead to significant economic disadvantages in industrial electrolysis. Low-impedance connections can generally be produced by short current paths, as mentioned, for example, in the DE-A-44 44 114. A low-impedance connection may also be obtained by a metal-metal contact, e.g. when the two or more metals are connected by soldering or welding. Therefore, the substrate of the GDE is optimally connected to a support structure of the electrolyser using a low-impedance connection made by welding or soldering. However, an effective seal also has to be achieved as well.